The forward Victor Moses has signed for Wigan from Crystal Palace on a three-and-a-half-year deal for a fee thought to be around £2.5m. Photograph: Tony Marshall/EMPICS Sport

Wigan's quest for survival has received a timely boost with the signing of the sought-after forward Victor Moses from Crystal Palace. According to the player's agent, Tony Finnigan, the player has signed a 3½-year deal for a fee thought to be around £2.5m after being sold by the administrator Brendan Guilfoyle, who was appointed last week to seek buyers for Palace's entire first-team squad.

But Roberto Martínez's other key signing this window, the defender Gary Caldwell, has the toughest job in the Premier League as the man charged with knocking some sense into Wigan Athletic's accident-prone backline. After three matches – and three goals conceded from set-plays in their last two games – he knows it will not be an easy job. He wants to use the move to impress the new Scotland manager Craig Levein. There must be easier ways.

Everton's manager David Moyes hailed Tim Cahill's powerful leap and header, six minutes from time, as proof that he is approaching his best form again. But headed goals are easier when opposition defenders are colliding with each other as if auditioning for a silent comedy.

Caldwell, signed from Celtic, was the closest defender when Cahill powered Leighton Baines's corner past Vladimir Stojkovic. When he unravels the mess with the help of TV replays he will discover that Cahill's marker, Maynor Figueroa, was expertly balked by two Everton players to allow the Australian to escape, and then stumbled into his own defenders, getting a stud in his foot for good measure, in a desperate attempt to recover lost ground.

This was the same Figueroa who received roars of approval from the crowd before kick-off for resisting Sunderland's overtures and signing an improved contract. But as Caldwell, clearly the defensive organiser in only his third game, barked his orders, it was Figueroa who most often adjusted his position a yard here, a yard there, like a recalcitrant schoolboy. There is some shouting to be done yet.

Caldwell attracted attention yesterday for politely suggesting that the November sacking of George Burley as Scotland manager was "good for the country", but it will be Wigan's shortcomings that will prey on his mind in the months ahead.

Only Hull have conceded more. The lure of Premier League football caused him to shun Middlesbrough, and his former manager Gordon Strachan, at the last minute – he must have been exceptionally close to signing because nobody spends two nights in a Teesside hotel without good reason.

Stojkovic, a giant Serbian goalkeeper, did not suggest himself as a reliable ally. He saved a mishit from Leon Osman with his knees, was wrongfooted when Louis Saha's drive deflected off Cahill and struck a post, and did not hold very much. He was also beaten by Marouane Fellaini's tap-in with 15 minutes left, but the effort was ruled out for offside.

Wigan are now only two points clear of the relegation positions and Martínez looks hot and bothered. "I am very angry and frustrated, we can't keep giving goals away from set plays," he said. "The person [Figueroa] responsible for marking Cahill didn't do his job. It's not complicated.''

Martínez made token claims that Wigan should have had a penalty six minutes into the second half, when Johnny Heitinga balked Charles N'Zogbia, but his heart was not really in it as TV replays had established that the challenge was outside the area, by a good inch.

Such niceties do not concern Moyes, who takes advantage wherever he can. He pronounced Caldwell's clumsy challenge against Saha early in the game as a "stonewall penalty'' and stared in that slightly disturbing way of his. Had Moyes been refereeing, Caldwell's afternoon could have gone wrong from the start. Things can always be worse than they are.